A study on the catalytic synergism between K and Pd in Co3Al(O) hydrotalcite-derived oxide catalysts used for soot combustion

• The Co3Al(O) catalyst promoted by K and Pd is highly efficient for soot oxidation.
• A prominent catalytic synergism between K and Pd on soot oxidation is observed.
• K–Co–O and K–Pd–O are identified as the main active phases for soot combustion.
• Three reaction pathways for soot combustion are revealed by in-situ FT-IR.

The K/Co3−xPdxAl(O) (x=0 or 0.02) and K/Pd/Co3Al(O) catalysts promoted by K and Pd were synthesized by using pH-constant co-precipitation and successive impregnation methods through the precursors of hydrotalcites. The physicochemical properties of these catalysts were systematically characterized by XRD, EXAFS, XANES, XPS, HRTEM, H2-TPR, soot-TPR and in-situ FT-IR. It is found that the catalyst K/Pd/Co3Al(O) prepared by successive impregnation displays the lowest characteristic temperatures including the initial and final combustion temperatures (Ti and Tf). The coexistence of K and Pd in K/Pd/Co3Al(O) or K/Co3−xPdxAl(O) can improve both the reducibility and the oxidizability of the catalysts, generating a prominent catalytic synergism on soot oxidation. K–Co–O and K–Pd–O have been identified as the main active phases in the catalysts simultaneously promoted by K and Pd. Based on the results of in-situ FT-IR three potential reaction pathways for soot combustion have been revealed, namely the direct soot oxidation, nitrate-enhanced soot oxidation and NO2-assisted soot oxidation. As compared with the conventional Pd/Al2O3 catalyst, the as-prepared K/Pd/Co3Al(O) catalyst possesses much higher thermal stability.